Yeah defining "random" is a lot harder than you think. And gibberish is in the eye of the beholder. The most important thing with a contact form is to make sure it cannot be abused as a spam relay. You need to be careful about how you create an email from user input because you might be susceptible to email header injection. It's helpful to sniff user input for line return characters in any data that finds its way into the email header.
As for screening the user message, you might consider a ban list -- a list of words or phrases that, if found in the message, will cause it to be discarded: e.g., "viagra" or "penis enlargement" or "canadian pharmacy".
It's also helpful to block problematic IP blocks. A lot of spam garbage hits one of my forums from Romania, China, Cyprus, etc. You can block large sections of those country by sniffing out the remote IP address.
